Michael Jackson may have been more prince than king of pop, a French doctor says in a new book alleging his wide-ranging voice resulted from a childhood chemical castration to fight acne.

"When he died, I realised that he was an unusual phenomenon," Alain Branchereau, an opera buff and professor of vascular surgery at Timone University Hospital in France's Mediterranean port of Marseille, told AFP.

"I said, 'That's the voice of a castrato!'."

After discussing the voice with his colleagues, including endocrinologists, Branchereau ended up with the theory of chemical castration through the synthetic anti-male hormone drug Cyproterone.

"When he was 12, Michael Jackson had acne. We know this, he spoke about it himself as a tragedy. What I think could have happened is that his people suggested this miracle treatment," the doctor said.

Cyproterone "blocks puberty, the voice can't mature," he said, adding that he had read around 20 books on the subject, studied photographs and spoken to specialists in dermatology, voice physiology, plastic surgery, urology as well as to a former singer with the Petits Chanteurs de Sainte-Croix boys choir.

The drug stops bodily hair and the larynx from growing and affects the bones, leaving the body with a slight frame but a large chest.

Once the treatment is finished, the patient "keeps a child's larynx all his life in a man's body," said Branchereau.

The doctor also noted that a male voice breaks during adolescence, becoming difficult to control. "But Michael Jackson never stopped singing," he said.

"An important part of my theory is this voice's exceptional character, which covers three octaves. But I haven't found any grown men's voices that cover three octaves."

Branchereau admitted that he had not contacted Jackson's family or friends for his book, "Michael Jackson, the secret of a voice," due out on March 9.

"We will never have proof," the doctor said. "Unless his entourage says something."